Add new extension to C++ plugin?

This ought to be simple, but I'm not seeing where/how to do it. I simply want to tell Qt Creator that filenames ending in ".doxy" should be treated as C++ files for editing purposes. The C++ editor preferences already know about "qdoc" files; it's just that I don't happen to be using qdoc.

Is there an easy way to do this, or is the knowledge of extensions buried deep in the source of Qt Creator?